The invention relates to multifilar helix antennas, for example described in WO 96/18220, specifically with isoflux radiation patterns. Satellites need antennas with various antenna pattern characteristics. The isoflux diagram is used in satellite antennas to illuminate the Earth in such a way that the power density is essentially constant regardless of where the ground station is located within the coverage area. The desired diagram is thus a conical rotationally symmetric diagram with the boresight in the nadir direction. The prefix iso-, from the Ancient Greek prefix ισoζ (“equal”), is here used to denote equal flux density. Assuming a spherical Earth with the radius Re and an orbit height of h, the following gain pattern gives the desired isoflux characteristic:
                                          G            ⁡                          (              θ              )                                            G            ⁡                          (                              θ                0                            )                                      =                                                                              [                                                                                    cos                        ⁢                                                                                                  ⁢                        θ                                            -                                                                                                                                  cos                              2                                                        ⁢                            θ                                                    -                                                                                    cos                              2                                                        ⁢                                                          θ                              0                                                                                                                                                                  cos                      ⁢                                                                                          ⁢                                              θ                        0                                                                              ]                                2                            ⁢                                                          ⁢              θ                        ≤                                          θ                0                            ⁢                                                          ⁢              sin              ⁢                                                          ⁢                              θ                0                                              =                                    R              e                                                      R                e                            +              h                                                          Eq        .                                  ⁢        1            
The formula does not correct for any atmospheric refraction, nor the minimum ground station elevation. In the WGS84 reference ellipsoid the Earth radius is 6378137 m.
FIG. 1 shows a schematic view over a satellite, Sat, and the earth approximated as a circle and a ground station, Gnd Stn.
The power pattern in the nadir direction is lower than at the edge of coverage (EOC), viz.
                                          G            ⁡                          (              θ              )                                            G            ⁡                          (                              θ                0                            )                                      =                                            [                                                1                  -                                      sin                    ⁢                                                                                  ⁢                                          θ                      0                                                                                        cos                  ⁢                                                                          ⁢                                      θ                    0                                                              ]                        2                    =                      1                          1              +                              2                ⁢                                                      R                    e                                    h                                                                                        Eq        .                                  ⁢        2            
A traditional all-metal quadrifilar helix antenna will suffer from a number of problems if the gain is increased by increasing the length of the helix radiator:                It will be difficult to maintain the correct beam properties, e.g. an isoflux pattern with good cross-polarization discrimination.        The conical beam peak tends to scan with frequency.        The back radiation and the slope outside the edge of coverage will be difficult to control.        The heat dissipated in the antenna will be difficult to conduct down to the base, thereby increasing the temperature at the top of the helix radiator and generating a thermal gradient along the helix.        The structural properties will be degraded (lowered mechanical eigenfrequency).        Various helix modes can interfere and yield resonances.        
There is a long-felt need to solve these problems to make it possible to increase the gain in existing helix antennas.
The main objective is thus to achieve an antenna that has a desired isoflux radiation pattern within the coverage region, along with suppressed radiation in the other directions. Also, the cross-polarization discrimination within the coverage is an important requirement.
There are several ways to achieve this type of patterns, e.g. large corrugated hats and helix antennas. The quadrifilar helix antenna (QFHA) is very efficient in this respect, and can enable a very small dimensional footprint, where the length of the antenna essentially determines the possible peak gain. However, the QFHA has other drawbacks discussed below why an improved antenna is desired.
Here, the nadir direction at a given point is the local vertical direction pointing in the direction of the force of gravity at that location. The direction opposite of the nadir is the zenith direction.